Conventionally, thermal sensors may be implemented using thin-layer technology, with the aid of temperature-dependent resistors, by preference Pt resistors, or with the aid of thermopiles, which are structures for utilizing the thermal voltage at transitions between two different metals or between metal and polysilicon. For this, these structures are applied onto a thin dielectric membrane whose low thermal conductivity allows, for example, changes in a temperature profile across said membrane to be sensed. This principle is applied, for example, in air mass sensors.
Because of their manufacturing process, both embodiments exhibit a high susceptibility to drift and therefore have the disadvantage that they must be very laboriously stabilized, since the changes in material properties over the service life bring about a drift of the sensor element. Even with such stabilization (e.g., tempering processes), this drift can in some cases still be enormous.
The temperature profile is set by way of thin-layer structures, e.g., resistance heaters made of platinum. Membrane manufacture is usually effected using a bulk micromechanical (BMM) process, i.e., all the material except for the membrane must be removed from the back side of the substrate by anisotropic etching, e.g., using KOH (potassium hydroxide). For example, in one conventional manufacturing process a 1-μm thick silicon membrane is generated on a wafer having a layer thickness of 360 μm, by way of an etching process, by removing 359 μm of the silicon layer.
Thermal sensors are additionally used, for example, in fingerprint sensors. Here the special sensor structures, heaters, sensing elements, and the like are applied onto a bulk substrate having low thermal conductivity and comparatively stable mechanical properties, e.g., ceramics, since the mechanical load in this application is considerably greater. The heat flux through the skin ridges is then detected and analyzed in spatially dependent fashion.
Thermally decoupled membranes are moreover also of interest in the field of gas sensing. Semiconductor-based gas sensors are typically based on an adsorption-related change in the resistance of metal oxide layers, or on a change in the potential of functional gate layer stacks of field effect transistors. To ensure effective dissociation of adsorbates and sufficiently short response times, however, and at the same time to prevent permanent contamination, these sensors must be operated at elevated temperatures, typically T>150° C. Depending on the application, membranes manufactured using complex process steps must also be used for thermal decoupling, in order, when necessary, to minimize heat output or enable rapidly pulsed operation. A method for manufacturing such a membrane is described, for example, in German Patent Application DE 102 00 40 24 285.2, “Microstructured component and method for the manufacture thereof.”
A further disadvantage of conventional systems results from their construction. Either high sensitivity or high mechanical stability can be achieved. With conventional methods, the two properties cannot be implemented simultaneously. A comparatively coarse spatial resolution, resulting from large planar structures of the thermopiles, Pt heaters, and sensor elements, may be disadvantageous.